The lighting of stage, theater, film and television has in the past typically been done with conventional lights. These lights have only limited capabilities and can generally perform only one function per light. This requires a great many lights to achieve the desired illumination effect.
Typically, lights are fixed in a specific location and can produce only one given colour. The shape of the beam that is projected is normally fixed as well. These elements of position, colour, and beam shape are determined when the lighting design is being carried out. When the lights are installed for the performance, they are adjusted to produce the desired effect.
The position of the light, or for that matter, the position of the image thrown by the light, is controlled by the position the light is mounted on the truss or other supporting member and the alignment of the light. The colour is controlled by placing a coloured material in the path of the light beam to produce the desired hue and saturation. The intensity of the light beam is generally determined by a power control device off stage and separate from the light itself. The beam shape is controlled by either focusing the beam at different distances to produce different degrees of beam divergence, or by placing a gobo or some other template in the path of the beam which alters the shape of the projected beam.
When a gobo is used to alter the shape of the light beam, the image is projected using varying degrees of focus to produce both sharp and soft projected images. The problem with this system is that in order to get a sharp image at the distance that you want to project, the image may not be the size that you desire due to the fixed focal length of the projecting lens.
While a large range of coloured materials exist for placement in the path of the light to alter the colour, these materials only change the hue and saturation of the light beam but not the colour temperature of the actual light source. This is very important for the film and television industry, where the cameras are very sensitive to variations in the colour temperature of the light source. A common problem is the filming of a scene in an environment where artificial lighting is required and a natural source of light already exists as well. The problem begins when the colour temperature of the two light sources are different from each other. This requires that one of the light sources be filtered to match both the colour temperature of the other light source and the film as well. This creates inefficient light sources and increased costs. Sometimes large areas such as windows need to be covered with the filter material. This is done to convert the light coming from a source on one side of the window into a compatible colour temperature with the light on the other side of the window.
A further problem with these coloured materials is that they work by absorbing all the wavelengths of light except the ones that are desired to produce that particular colour. The result is that the filters absorb the unused wavelengths of light and convert them into heat, typically melting or discolouring the filter from the heating effects. This means that they have a short life span and need constant replacing.
A more obvious problem of this form of light colouring is that you can only produce one colour from the coloured material.
The light sources also produce a substantial amount of heat. This intense heat from the lights is very unpleasant for the performers on stage and a constant problem in the close up world of television and film, where the heat from all the lights can spoil makeup and other heat sensitive effects that have been created.
Since the lights are fixed in a particular location, they do not possess the ability to be pointed at another location during the show. This increases the number of lights that need to be used during the performance.
A partial solution to some of the above problems is described in U.S. Pat. No. 4,392,187 to Bornhorst. His system includes a light which can produce a number of colours and vary the beam divergence and position of the projected image. In his system, the colours are produced by introducing a number of coloured filters into the path of the light beam, that instead of absorbing the unused portion of the light, reflects it off the surface of the filter. This helps to eliminate some of the heating effects that occur in the filters and increases their life span. By adjusting the position of these filters in the path of the light beam, a number of colours can be achieved. The heat from the light source still escapes from the light and lands on the stage, still causing discomfort and heating the objects in the path of the light. While this invention can produce a range of colours, this method cannot produce a continuous range of colours.
A method of producing a continuous range of colours is described in U.S. Pat. No. 4,535,394 to Dugre. His system uses three primary coloured light sources, which he combines using two dichroic mirrors into a single light beam.
While the basic optical idea is feasible, it is inefficient due to the extra filtering of the light sources that is required to produce the three primary coloured light sources. If the filtering is performed using the coloured materials that are used on conventional lights, then this system will fall prey to the same heating effects that ruin these materials on the conventional lights. This would mean that the light would fail before the performance was finished and you would constantly need to replace the coloured material. Although not specified in the patent, it is more likely that the same sort of dichroic filters that are used in the Bornhorst invention previously described, would be used here because of the ability to reflect unwanted wavelengths, which cuts down on the heating of the filters from this waste light. The problem with these dichroic filters is that they are heat sensitive. The heating effects from the high power light sources will cause a temperature induced colour drift in the primary filters. This will vary depending on the present intensity of the individual lights. This will make it difficult, if not impossible to accurately produce a desired colour at any given time due to the unknown degree of colour shift that has occurred at that point in time.
If the light sources are left on constantly, the colour shift can become quite substantial.
The heat will also cause aging of the filters, which will show up as a permanent shift in colour. This will necessitate the frequent replacing of these rather expensive filters.
The means of control of the intensity of the three primary light sources, and the indecisiveness of the exact amount of the three primary colours that are being added, makes the control of the colour temperature of the light, not to mention, the exact colour being produced, impossible. This form of controlling the light will allow only course changes in colour, and would not achieve much more of a range of colours than the Bornhourst invention previously mentioned. However, since an additive method of colour generation is being used, rather than the previously mentioned method of discrete filters, a continuous range of colour can be produced.
A further problem with the Dugre invention is that the optical systems he has described will not produce a single clean coloured light beam. The length the light travels from each of the three light sources is different, and therefore the angle of divergence of the three light beams will be different. This causes the composite beam to appear as three overlapping cones of light when it reaches the stage. Any shadow produced on the stage by a beam of light from this optical system will not produce one distinct shadow, as would a single coloured beam, but rather a number of separate and differently coloured shadows behind the performer on stage. This is a distracting side effect and not really suitable for use in illumination of stages or other types of performances. A true single beam of light would produce only one clean shadow, with no colour separation occurring.
The heat from the light sources is still able to reach the stage in this optical design causing all the above mentioned problems.
None of this known art teaches a light that has a continuously variable colour temperature, as well as continuously variable colour, which can be repeatedly produced, and further, which can produce a variable sized image, that can be focused over a large range of distances, and carries no heat in the light beam falling on the stage.